Asphalt may be characterized as an organic cementitious material in which the predominant constituents are bitumens as they may occur in nature or as they may be produced as byproducts in petroleum refining operations. Asphalt can generally be characterized as a dark brown or black solid or highly viscous liquid, which incorporates a mixture of paraffinic and aromatic hydrocarbons as well as heterocyclic compounds containing Group 15 or 16 elements, such as nitrogen, oxygen or sulfur.
Asphalts have many industrial applications involving use as paving or road surfacing materials. Perhaps the most widespread use of asphalt compositions is in road surfacing and paving applications. The asphalt may be used alone, such as where it is applied to the surface of an existing paving structure, or it may be used as an aggregate composition in which the asphaltic base material is mixed with an aggregate, typically in an amount of 4-10 wt. %. The asphalt material can be modified through the use of polymers to produce so-called polymer-modified asphalts. Polymer-modified asphalts or “PMA” function to provide improved characteristics as a paving material.
In addition to polymers, it is a conventional practice to incorporate scrap rubber particles into an asphalt matrix material to form asphalt paving or sealing materials. Such scrap rubber particles are referred variously as ground tire rubber (GTR) or crumb rubber and can include materials recovered from tire carcasses, reclaimed tire treads and the like. The asphalt base material incorporating such crumb rubber particles can be of any suitable type such as derived from petroleum refining operations and include aliphatic and aromatic hydrocarbons and heterocyclic compounds, including asphaltenes and malthenes of fairly high molecular weight. Aggregate particles as described above, ranging from sand to crushed rock the size of perhaps ¼-½ inch can be incorporated into the asphalt base material. The asphalt binder aggregate concrete formulation can be used as a concrete base for roads. Alternatively, a blend of asphalt can be applied as a sealer coat on top of existing road paving. Such sealing coats typically may be a thickness of ⅛-¼ inch and may incorporate aggregate materials, or relatively finely ground aggregate materials can be dispensed on the sealer coat while it is hot, immediately after its application or after allowing the sealer to set for a period of a few hours or days. Yet another procedure for providing an asphalt/aggregate road composition involves depositing a layer of the aggregate material on a substrate surface such as a road bed or the like and thereafter depositing the asphalt binder material on the layer of aggregate to provide an asphalt/aggregate road surfacing composition.
As noted previously, polymers can be added to asphalts to improve physical and mechanical performance properties. Such polymers include elastomeric-type polymers such as butyl, polybutadiene, polyisoprene or polyisobutene rubber, ethylene/vinyl acetate copolymer, polyacrylate, polymethacrylate, polychloroprene, polynorbomene, ethylene/propylene/diene (EPDM) terpolymer and advantageously a random or block copolymer of styrene and a conjugated diene, such as butadiene or isoprene. The modified asphalts thus obtained are referred to variously as bitumen/polymer binders or asphalt/polymer mixes. Modified asphalts and asphalt emulsions can be produced utilizing styrene/butadiene-based polymers to provide raised softening points, increased viscoelasticity, enhanced force under strain, enhanced strain recovery and improved low temperature strain characteristics.
An important factor in asphalt/aggregate paving materials is the degree of adhesion between the asphalt and the aggregate in the paving material. If good adhesive characteristics are not present, exposure of the asphalt/aggregate paving formulation to traffic and weather conditions ultimately results in stripping of the asphalt and an undesirable separation of the asphalt and aggregate material with crumbling and disruption of the paving surface. Another important consideration is the degree of contact or coverage of the asphalt binder material over the aggregate particles. If there is good contact of the asphalt over the surfaces of the aggregate particles, the resulting composition can be expected to have a high degree of integrity. However, if coverage is spotty and incomplete, the asphalt/aggregate composition will show poor integrity under heavy vehicular traffic, resulting in crumbling of the asphalt/aggregate concrete with formation of potholes and the like.